Lost wax casting, now called investment casting, is a casting process with less or no cutting. It is an excellent process technology in the foundry industry and its application is very extensive. It is not only suitable for the casting of various types and various alloys, but also produces castings with higher dimensional accuracy and surface quality than other casting methods, and even castings that are difficult to cast by other casting methods, which are difficult to cast, are not easy to process. Can be cast by investment casting.
Investment casting is developed on the basis of ancient wax mold casting. As an ancient civilization, China is one of the countries that used this technology earlier. In the hundreds of years before BC, the ancient Chinese working people created this lost wax casting technique, which was used to cast a variety of fine patterns and texts. The products of Zhong Ding and utensils, such as the tomb of Zeng Houyi at the Spring and Autumn Period. The base of Zenghouyi's tomb is a pair of intertwined dragons. They are connected end to end and are staggered up and down to form a multi-layered cloud pattern in the middle. These patterns are difficult to manufacture by ordinary casting techniques and are cast by lost wax method. The process can take advantage of the fact that paraffin has no strength and is easy to engrave. With ordinary tools, it can engrave the paraffin material craftsmanship like the desired Zenghou Yi tomb, and then add the pouring system, paint, dewaxing and pouring. Can be exquisite Zenghou Yi tomb.
The practical application of modern investment casting methods in industrial production was in the 1940s. At that time, the development of aviation jet engines required the manufacture of heat-resistant alloy parts such as blades, impellers, nozzles, and the like, which were complicated in shape, size, and smooth in surface. Since the heat-resistant alloy material is difficult to machine, the shape of the part is so complicated that it cannot be or is difficult to manufacture by other methods. Therefore, it is necessary to find a new molding process, so that the lost wax casting which has been handed down from ancient times is passed through the materials and processes. Improvements, modern investment casting methods have gained important development on the basis of ancient techniques. Therefore, the development of the aviation industry has promoted the application of investment casting, and the continuous improvement and improvement of investment casting has also created favorable conditions for the aviation industry to further improve its performance.
In the 1950s and 1960s, China began to apply investment casting to industrial production. Subsequent developments in this advanced casting process have been widely adopted in the manufacturing industries of aviation, automobiles, machine tools, ships, internal combustion engines, gas turbines, telecommunications equipment, weapons, medical devices and cutting tools, as well as in arts and crafts. Manufacture of goods.
The so-called investment casting process, simply by using a fusible material (such as wax or plastic) to make a fusible model (referred to as investment or model), coated with several layers of special refractory coating, dried and hardened After forming a monolithic shell, the model is melted from the shell by steam or hot water, and then the shell is placed in a sand box, filled with dry sand molding around it, and the mold is placed in a roaster and subjected to high temperature roasting. (When a high-strength type shell is used, the demolded shell can be directly fired without styling), and the mold or shell is fired, and molten metal is cast therein to cast.
Investment castings have high dimensional accuracy, generally up to CT4~6 (sand casting is CT10~13, die casting is CT5~7). Of course, due to the complicated process of investment casting, there are many factors affecting the dimensional accuracy of castings, such as mold. The shrinkage of the material, the deformation of the investment mold, the change in the amount of the shell during heating and cooling, the shrinkage of the alloy, and the deformation of the casting during solidification, etc., so the dimensional accuracy of the ordinary investment casting is high, but Consistency still needs to be improved (the dimensional uniformity of castings using medium and high temperature waxes is much higher).
When the investment mold is pressed, the molding having a high surface finish of the cavity is used, and therefore, the surface finish of the investment mold is also relatively high. In addition, the shell is made of a refractory coating made of a special binder and a refractory material which is resistant to high temperature, and is coated on the investment mold, and the inner surface of the cavity which is in direct contact with the molten metal has a high surface finish. Therefore, the surface finish of the investment casting is higher than that of the general casting, and generally reaches Ra.1.6~3.2μm.
The advantage of investment casting is that the investment castings have high dimensional accuracy and surface finish, so the machining work can be reduced, but only a small amount of machining allowance is required on the parts, even some castings only Leave the grinding and polishing allowance and use it without machining. It can be seen that the investment casting method can save a lot of machine tool equipment and processing man-hours, and greatly save metal raw materials.
Another advantage of the investment casting process is that it can cast complex castings of various alloys, particularly casting high temperature alloy castings. For example, the blade of a jet engine, its streamlined profile and cooling cavity, can hardly be formed by machining. The production by the investment casting process can not only achieve mass production, but also ensure the consistency of the castings, and avoid the stress concentration of the residual knives after machining.